Saturday, April 24, 2010

Craters on the Schrödinger pyroclastic cone

LROC Narrow-Angle Camera (NAC) closeup of clustered craters on the lip of the Schrödinger pyroclastic cone, one of the NASA Constellation Regions of Interest (ROI). Although believed to be relatively young, these craters have a subdued appearance, a texture smoothed by micrometeor 'gardening' typical of older lunar surfaces) because they formed in loose pyroclastic material. NAC Frame M108313384R, this view is 785 meters across [NASA/GSFC/Arizona State University].

Lisa Gaddis
LROC News System

How old is the Moon? How old are the craters and basins, and the dark mare that fills many of them? When did lunar volcanism begin and end? These and many other questions about the Moon have been asked for millennia, and one of our best means for answering these questions is by simply counting the number impact craters on a particular feature.

This NAC frame (785 m across) shows a cluster of small impact craters on the rim of a cone-shaped pyroclastic deposit in the floor of Schrödinger Basin. The largest crater in the view is about 690 meters across. Schrödinger Cone is one of the largest single-vent formations yet observed on the Moon; crater counts by Eugene Shoemaker, et al. (1994) suggested it may be less than a billion years old -- ancient but still young by lunar standards!

Mosaic of Clementine (1994) UVVIS (ultraviolet-visible spectrophotometry) images (750-nm band) of Schrödinger Basin (312 km diameter). In addition to the prominent, darker cone-shaped feature (arrow) Schrödinger has an inner ring of mountains partially encircling the basin floor (a ‘peak ring complex’) and a network of radial and concentric fractures. The cone is a likely volcanic vent situated on a northeast trending floor fracture, and it has a 4.5 km x 8.6 km vent surrounded by dark, explosively emplaced (pyroclastic) material and a low rim. The Schrödinger Vent is one of the most distinctive single-vent cones observed on the Moon, resembling the ‘dark halo craters’ like those on the floor of Alphonsus. Projection is polar stereographic, centered at -75.0°S, 132.0°E [NASA/DOD/LROC/ASU].

Impact cratering is one of the few processes affecting all planetary surfaces, including the surface of our own Earth. To understand the relative ages of craters, basins, and surface deposits on the Moon, we study its impact craters. More specifically, we count the number and measure size, shape, and distribution of craters and compare those from place to place on the Moon.

We know that older surfaces have more craters (because they have been exposed to impacts for longer than younger surfaces) and we’ve learned that over time newer craters begin to erase the older ones. We call this ‘equilibrium’. If we relate our crater counts to models of the number of things (such as meteors, asteroids and comets) roaming around in space available to create those craters, we can begin to understand the relative ages of features and units on the Moon. Such relative ages have been tied to real or absolute ages at the few sites from which samples have been returned by the Apollo and Luna missions.

Kaguya (SELENE 1) multi-band imager (MI) compositional and morphological study of the Schrödinger pyroclastic formation (75.3°S, 139.1°E) [JAXA/SELENE].

Studies of lunar impact craters have worked well in areas such as ‘typical’ mare and highlands units where the properties of surface soils and rocks are reasonably well understood. Are crater counts reliable on pyroclastic deposits, where the soil is formed from loose, relatively unconsolidated ash. Craters formed in such material can appear older than they are because they tend to be shallower, with lower rims and subdued ejecta blankets. The loose material in a pyroclastic deposit appears to both alter the original appearance of an impact crater and make the process of degradation more efficient. This means that craters disappear faster than they do on harder surfaces, such as the mare basalt. If corrections for the type material that craters form in are not applied, then relative age estimates are in error. Such is likely the case with the craters in this cluster on the rim of the volcanic cone in Schrödinger basin.

Crater counts for the region around and within Schrödinger basin by Shoemaker et al. (1994) indicate that it is likely the second-youngest basin on the lunar surface. Schrödinger and its interior deposits are of interest in future lunar exploration in part because pyroclastic deposits have economically valuable components such as iron and titanium.

Moving north (top) in a polar orbit, Japan's Kaguya took extensive HDTV of the lunar far side, including this still showing the Schrödinger Basin interior. The low and relatively darker profile of the pyroclastic dome encircling the vent is right (east) of the image center [JAXA/SELENE].

For more information on LROC's observation campaign for the Constellation Program regions of interest read this Lunar and Planetary Science Conference abstract, and visit the LRO Science Targeting Meeting website (look for the baseball card summary sheets for each site: part 1, part 2).

Explore the Schrödinger Constellation region of interest for yourself!

For more information, see: Shoemaker, E.M., M.S. Robinson, and E. Eliason (1994), The South Pole of the Moon as Seen by Clementine, Science, v. 266, pp. 1851-1854.

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